Supported zeolite and MOF

Supported Zeolite and MOF Molecular Sieve Membranes Preparation, Characterization, Application... 

The concept of this widely used technique consists in the decoupling of (i) nucleation and seed formation at high supersaturation, and (ii) growth of the seed crystals to a continuous layer at low supersaturation. Usually, in the crystal growth step a new nucleation can be avoided and only the seeds grow to the molecular sieve layer. Therefore, the layers obtained by secondary growth are less polycrystalline. If the support surface was covered by a homogeneous and dense seed layer, relative thin zeolite and MOF layers can be obtained. 

There is a similarity between zeolites and MOF membrane preparation. For the synthesis of an MOF membrane, all the tools such as seeding, microwave heating, ceramic porous supports, and intergrowth-supporting additives, which are used for zeolite membranes, can be applied. Thus, the synthesis of MOFs can be achieved in... 

In addition to their beauty, many of the described nanovessels also show interesting endo/exo chemistry ( inside and outside ). In the interior, species can be bound, and highly reactive intermediates can be stabilized, or chemical reactions supported or catalyzed. In the latter case, unusual reactivity or selectivity might be observed. Thus, container molecules act as homogeneous equivalents of heterogeneous porous materials like zeolites or MOFs. 

Finally, in an approach analogous to that used for porous carbons and zeolites, highly robust MOFs have been used as surface supports for metal atoms and clusters. An example here is the chemical vapour deposition of various metals into MOF-5, yielding materials classified as metal MOF-5 for which the nature of metal inclusion and the extent of exogenous loading is currently unknown. Of these, Cu MOF-5 is active in the synthesis of methanol from syngas and Pd MOF-5 catalyses the reduction of cyclooctene by hydrogen. ... 

Zeolites, which form one great family of crystalline porous materials, are broadly used in catalysis (petrochemicals cracking). However, postmodification ofmicrop-orous zeolites is limited to cation exchange or silanation. In addition, zeolites also suffer a drastic limitation in their small pore sizes. Among other porous materials, MS materials, such as MCM-41 and SBA-15 (SBA, Santa Barbara amorphous) [66, 67], are widely used as adsorbents or catalyst supports however, unlike the highly ordered MOFs, their walls are amorphous and thus exhibit relatively disordered surface hydroxyl group distribution [68]. In addition, the diversity of MS materials is limited in terms of composition and porous structure. 

In the MOF PIZA-3 (PIZA, porphyrinic Illinois zeolite analog), Mn(III) is found both in the porphyrin struts and as a structural metal node. The framework is structurally stable and is used for the oxidation of cycUc alkanes and alkenes with iodosylbenzene or peracetic acid as the oxidant [117]. Reaction is found to take place at the outer surface, which is justified by the authors by the unfavorable hydrophilic properties of the pore interior. Yields were similar to those obtained with homogeneous Mn(III) porphyrin systems or those immobilized inside inorganic supports as heterogeneous catalysts. Less than 0.1 mM of metalloporphyrin or degradation products were observed in the reaction mixtures, with no loss of oxidation activity observed in a second run when peracetic acid was used. 

Eddaoudi and coworkers developed a strategy for heterogenerization of porphyrin catalysts by impregnating free-base porphyrins into the pores of MOFs, and the catalytic function was realized by subsequent met-alation with active metal ions of Mn, Cu, Zn, and Co via postsynthesis. They proved that the H2RTMPyP platform was efficient in heterogeneous cyclohexane oxidation. Mn-RTMPyP oxidized cyclohexane to cyclo-hexanol/cyclohexanone in 91.5% yield with turnover number of 23.5, based on the amount of oxidant consumption, which is noticeably higher than other systems of supported metalloporphyrins on zeolites or mesoporous silicates. 

The challenge can be described as follows To get a thin and perfect molecular sieve (zeolite, MOF) layer, nucleation and crystal growth has to start on as many positions on the support as possible. The more nucleation centers on the support surface, the thinner can be the dense... 

---